Purifying apparatus

ABSTRACT

This invention relates to a method of purifying gases which are polluted by hydrocarbons and the like in an apparatus having two reactors. Each reactor contains a catalyst bed, a ceramic bed and a border layer located between the two beds. The polluted gas is forced to stream alternately through the first reactor and the second reactor (clock-wise streaming) and the second reactor and the first reactor, respectively (counter clock-wise streaming). The purification of said gas occurs when the hydrocarbons in the gas are oxidized to carbon dioxide and water in the catalyst beds. The temperature of the gas streaming through the upper portion of the reactors is measured. When the temperature exceeds a certain level, a portion of the gas is taken out from the apparatus during the passage over the border layer of the second reactor if said gas is streaming in a clock-wise direction, and a portion of said gas is taken out from the apparatus during the passage over the border layer of the first reactor if said gas is streaming in a counter clock-wise direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention related to a method of purifying gases which are pollutedby hydrocarbons and the like. The invention also relates to an apparatuswhich is usable for carrying out the method.

2. Description of the Prior Art

Authorities and the public have lately strongly increased the demandsfor a reduction of outlets of substances polluting the environment. Suchsubstances can be hydrocarbons in solvents which are admixed to theventilation air from an industrial plant.

There is today in the market an apparatus for purifying gases, forinstance ventilation air having an admixture of hydrocarbons, thepurification being made by catalytic combustion so that the hydrocarbonsdamaging for the environment are transformed to carbon dioxide andwater. This apparatus comprises two reactors, each of which having acatalyst bed and a ceramic bed. The combustion reaction is exothermic,i.e. it emits heat. If the concentration of pollutions, i.e. in thiscase the content of hydrocarbons, is high enough, the combustion processbecomes self-supporting, which means that no external energy needs to besupplied.

Heat emitted during the combustion reaction increases as the content ofpollutions increases in the gas that shall be purified. This means thatthe temperature in the purification apparatus can be so high that thematerial is damaged. Owing to that the apparatus known in the market haslimitations regarding the possibility to manage purification of gaseshaving high contents of pollutions.

According to known technique there has been an attempt to solve thisproblem by making possible to take out gas to atmosphere at an areabetween the two reactors (see for instance EP-337 143). In order thatthis shall be acceptable from an environmental point of view, however,such a channel going out to the atmosphere has to be provided with aseparate catalyst. This solution is of course both complicated andexpensive.

This invention intends to offer a solution of the problem of purifyinggases having high contents of pollutions, which solution isuncomplicated, relatively spoken, and attractive from an expendituralpoint of view. This has been made possible by a method of the kindmentioned by way of introduction, which is characterized by the momentsmentioned in the claims.

SUMMARY OF THE INVENTION

This invention relates to a method of and apparatus for purifying gaseswhich are polluted by hydrocarbons and the like. The apparatus has tworeactors, each reactor comprising a catalyst bed, a ceramic bed and aborder layer lying between the two beds. According to the invention, thegas that shall be purified is forced to stream alternately through thefirst reactor and the second reactor (clock-wise streaming) and thesecond reactor and the first reactor, respectively (counter clock-wisestreaming). The gas is purified when the hydrocarbons in the gas areoxidized to carbon dioxide and water in the catalyst beds. Thetemperature of the gas streaming through the upper portion of thereactors is measured and when the temperature exceeds a certain level, aportion of the gas is taken out from the apparatus during the passageover the border layer of the second reactor if the gas is streaming in aclock-wise direction, and a portion of the gas is taken out from theapparatus during the passage over the border layer of the first reactorif the gas is streaming in a counter clock-wise direction.

A preferred embodiment of an apparatus according to the invention,useable for carrying out the method shall be described more closelybelow with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of the dual reactor embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawing is shown there the new apparatus. This onecomprises two reactors 1, 2 which are placed at a distance from eachother and are preferably cylindrical. The two reactors 1, 2, which arepreferably vertically arranged, are at their upper portions connected bya pipe means 3. Each reactor 1, 2 has a catalyst bed 4, 5 and a ceramicbed 6, 7. The catalyst beds are intended to function as purificationmeans for the gas, whereas the ceramic beds are intended to function asheat storage means. Furthermore, each catalyst bed 4, 5 is placed on thetop of the ceramic bed 6, 7. An electric heater 8, 9 is arranged in theupper portion of each reactor 1, 2. External, electric energy by thesetwo electric heaters can be supplied to the gas which is intended tostream through the reactors. Of course, it is possible to use a gasburner instead of electric heaters for heating the gas. The lowerportion of each reactor 1, 2 is connected by a pipe means 10, 11 with anexchange valve 12, by means of which it is possible to control theinstreaming gas flow so that it first passes the first reactor 1 andthen the second one 2, alternately, first the second reactor 2 and thenthe first one 1. Due to that fact the ceramic beds 6, 7 alternatelyfunction as heat absorbers and heat emitters.

The exchange valve 12 has an entrance 13 for polluted gas and two exits14, 15 for purified gas. The exchange valve 12, however, is so designedthat when the one exit 14 is open, is the other exit 15 closed and viceversa. The two exits 14, 15 are connected to one and the same conduit 16which leads the purified gas out to the atmosphere.

The entrance 13 of the exchange valve 12 is connected with a conduit 17which is intended to lead the polluted gas from an emission source 18via a shut-off valve 19 and a fan means 20 to the exchange valve 12 andfurther into the reactors 1, 2. Between the conduits 16, 17 there is aarranged a bypass-valve 21.

The apparatus further has a conduit 22 connecting the two reactors 1, 2with each other. This conduit 22 is connected to each reactor 1, 2approximately at the transition area between the catalyst bed 4, 5 andthe ceramic bed 6, 7. The conduit 22 is connected with the conduit 16 tothe atmosphere by a further connection conduit 23. Due to that factthere is created a conduit portion 22a connecting the one reactor 1 withthe connection conduit 23 to the atmosphere and a conduit portion 22bconnecting the other reactor 2 with the connection conduit 23. In eachof these two conduit portions 22a, b is a shut-off valve 24, 25arranged. These valves are so arranged that when one 24 is open, is theother one 25 closed and vice versa. According to the preferredembodiment shown in the drawing the conduit 22 is connected torespective reactor in the border layer 26, 27 between the catalyst bedand the ceramic bed. Due to that fact an optimum purification isachieved also of the gas which is taken out form the apparatus via theconduits 22, 23.

The apparatus functions in the following way:

When starting the system the valve 19 close to the emission source 18 isclosed at the same time as the bypass-valve 21 is kept open. The fanmeans 20 transports air via the conduit 17, the exchange valve 12 andthe conduit 11 into the second reactor 2 (with the position the exchangevalve 12 has in the drawing). The air passes the electric heaters 9 and8 where it is heated and further through the catalyst bed 4 and theceramic bed 6 of the first reactor 1.

In the ceramic bed 6 the heat of the air, which was absorbed from theelectric heaters, is accumulated. After a counter-clockwise streaming inthe apparatus during a certain time, the position of the exchange 12 ischanged so that the air is allowed to stream in a clockwise direction,i.e. the air is allowed to pass the apparatus via the first reactor 1and the second reactor 2. The air absorbs the accumulated heat amount inthe first ceramic bed 6 which at the same time is cooled. Furthermore,the air gets a heat addition from the electric heaters. During thepassage through the second reactor 2, the heat amount of the streamingair is emitted to the second ceramic bed 7 which is accordingly heated.Thus, the ceramic beds 6, 7 alternately function as heat absorbers andheat emitters.

As the alternate streaming continues, the temperature of the airincreases. At a certain temperature the valve 19 close to the emissionsource is opened at the same time as the by pass-valve 21 is closed.This fact has the consequence that polluted gas from the emission source18 is conducted into the purification apparatus and is allowed to streamthrough the purification apparatus comprising the reactors 1, 2. In thisphase the streaming of the gas is made alternately in acounter-clockwise and a clockwise direction.

The catalytic combustion process starts at a temperature of about 300°C. In that connection hydrocarbons of the polluted gas are oxidized tocarbon dioxide and water which can be let out to the atmosphere.

As has been mentioned previously, the combustion process is exothermic,i.e. energy is made free during the oxidation to carbon dioxide andwater. This energy amount made free can be sufficient for maintaining astable temperature in the apparatus. However, the energy amount madefree increases with an increased degree of pollution of the gas, i.e.with an increased content of hydrocarbons in the gas. This fact can leadto severe material problems in the apparatus if the degree of pollutionis high in the gas that shall be purified.

According to the invention the problem is solved by the fact that heatis deaerated via the conduit 22a alternatively 22b and the conduit 23out to the atmosphere. This is made in that way that the temperature ofthe gas being purified is recognized in the upper portion of theapparatus. If the temperature exceeds a certain value, let us say 500°C., the valve 24 is opened during a counter-clockwise streaming (thevalve 25 is closed), while the valve 25 is opened during a clockwisestreaming (the valve 24 is closed). Due to that fact hot gas can streamout via the conduits 24a, b and 23 so that the temperature in theapparatus is kept on an acceptable level. Thanks to the fact that theconduits 22a, b have such a connection to the two reactors as has beenmentioned previously, the gas is allowed to pass also the catalystnumber 2 in the circulation system, before the gas is let out to theatmosphere via the conduits 22a, b and 23. Due to that fact also thedeaerated gas becomes effectively purified.

Thus, this new apparatus functions effectively when purifying gaseswhich are strongly polluted by hydrocarbons. The apparatus is also verysuitable to be used when cleaning gases in which the content ofhydrocarbons strongly varies.

The invention has been tested in several different plants and it hasappeared that it functions very effectively. During one of the tests areport was made of the measurement of the degree of purification duringpurification of hydrocarbons (propane). In that connection the followingmeasuring method was used:

FID-ANALYSATORJUM 3-300, calibrated to propane and nitrogene. Thefollowing result was received:

    ______________________________________                                        Measur-                                                                              Meas-   Measur-  Average Degree of                                                                             Temper-                               ing    ured    ing      content of                                                                            purifica-                                                                             ature                                 point  airflow period   pollutions                                                                            tion    area                                  ______________________________________                                        Intake 14.600  13,43 →                                                                         1619 ppm*.sup.)                                       air    Nm.sup.3 /h                                                                           16,09                                                          Exhaust                                                                              14.600  13,43 →                                                                         76,5 ppm*.sup.)                                                                       95,3%                                         air    Nm.sup.3 /h                                                                           16,09                                                          Deaera-                                                                              4.500   13,43 →                                                                         76.5 ppm*.sup.) 500-                                  tion   Nm.sup.3 /h                                                                           16,09                    450° C.                        ventila-                                                                      tion                                                                          ______________________________________                                         *.sup.) 1619 ppm corresponds to 3,52 g/Nm.sup.3.                              76,5 ppm corresponds to 0,166 g/Nm.sup.3.                                

The maximum values of the degree of pollution were 3.500 ppm (7, 6g/Nm³).

As appears from the table 4.500 Nm³ /h were deaerated. This correspondsto a heating effect of 750 kW, which accordingly could be utilized forheating purposes.

As has been mentioned previously the energy amount made free increaseswith an increased degree of pollution in the gas. The deaerated heatenergy amount, based on a heating effect of 750 kW according to thetest, would accordingly correspond to an extra degree of pollution ofapproximately 5 g/Nm³. This means that the working area for a plantprovided with the new invention can be enlarged from a degree ofpollution of about 3 g/Nm³, which is valid for purification inconventional plants, to a degree of pollution of about 8 g/Nm³ withoutany deterioration of the degree of purification.

The invention is of course not limited to what has been described butcan be modified within the scope of the following claims.

What is claimed is:
 1. A method of purifying a gas polluted byhydrocarbons in an apparatus having two reactors, each reactor includinga catalyst bed and a ceramic bed with a border layer therebetween, saidmethod comprising the steps of:alternately streaming said gas in aclock-wise direction through the first reactor and then through thesecond reactor, and streaming said gas in a counter clock-wise directionthrough the second reactor and then through the first reactor, whereinthe hydrocarbons are oxidized to carbon dioxide and water as thehydrocarbons pass over the catalyst beds; monitoring the temperature ofsaid gas streaming through an upper portion of the reactors; removing aportion of said gas streaming in the clock-wise direction as said gaspasses over the border layer lying between the catalyst bed and thecorresponding ceramic bed of the second reactor when said gas exceeds acertain temperature; and removing a portion of said gas streaming in thecounter clock-wise direction as said gas passes over the border layerlying between the catalyst bed and the corresponding ceramic bed of thefirst reactor when said gas exceeds a certain temperature.
 2. Anapparatus for purifying a gas polluted by hydrocarbons; said apparatuscomprising:two reactors positioned at a distance from one another; pipemeans connecting the two reactors together; the first reactor includinga first catalyst bed connected to a first ceramic bed; a first borderlayer lying between the first catalyst bed and the first ceramic bed;the second reactor including a second catalyst bed connected to a secondceramic bed; a second border layer lying between the second catalyst bedand the second ceramic bed; and conduit means for venting said gas fromeach reactor to the atmosphere, the conduit means being connected toeach reactor at the border layers so that said gas may be vented to theatmosphere from the border layer of each reactor.
 3. The apparatusaccording to claim 2, wherein the conduit means comprises a plurality ofconduits;each conduit including at least two shut-off valves; whereinone of the shut-off valves opens and closes the connection between thefirst reactor and the atmosphere and the other shut-off valve opens andcloses the connection between the second reactor and the atmosphere; theshut-off valves being operated so that when one valve is open, the othervalve is closed.
 4. The apparatus according to claim 3, wherein thefirst conduit connects the two reactors together;the first conduit beingdivided into two conduit portions; the first conduit portion connectingthe first reactor with the second conduit; the second conduit portionconnecting the second reactor with the second conduit; and one of theshut-off valves being located in the first conduit portion and the othershut-off valve being located in the second conduit portion.
 5. Theapparatus according to claim 4, wherein each reactor is verticallyarranged so that each catalyst bed is located on top of eachcorresponding ceramic bed; and wherein each conduit portion connects toeach reactor in the corresponding border layer.